Within the realm of classical systems, most of the work on FRs and
other aspects of ST (such as the recently discovered thermodynamic
uncertainty relation (TUR) [1]}, [2]} has been
devoted to fluctuating systems exhibiting normal
diffusion. Considering, for simplicity, 1D motion of a Brownian
particle in a suspension, 'normal' diffusion implies that the
mean-squared displacement (MSD) \(\langle x^2(t)\rangle \)  (with \(x\) 
being the distance travelled at time \(t\) , averaged over an
ensemble of particles) increases linearly in \(t\)  at long
times. Such processes are typically modelled by a conventional
Langevin equation (LE) involving white noise, which is related to the
friction via the (2nd) fluctuation-dissipation relation
[3]}, .
